Problems with fogging have plagued the photographic industry from its inception. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. Fog can be defined as a developed density that is not associated with the action of the image-forming exposure, and is usually expressed as "Dmin", the density obtained in the unexposed portions of the emulsion. A density, as normally measured, includes both that produced by fog and that produced by exposure to light. It is known in the art that the appearance of photographic fog related to intentional or unintentional reduction of silver ion (reduction sensitization) can occur during many stages of preparation of the photographic element including silver halide emulsion preparation, (spectral) chemical sensitization of the silver halide emulsion, melting and holding of the liquid silver halide emulsion melts, subsequent coating of silver halide emulsions, and prolonged natural and artificial aging of coated silver halide emulsions.
Several methods have been employed to minimize this appearance of fog. Mercury containing compounds, such as those described in U.S. Pat. Nos. 2,728,663; 2,728,664; and 2,728,665, have been used as additives to combat fog. Thiosulfonate and thiosulfonate esters, such as those described in U.S. Pat. Nos. 2,440,206; 2,934,198; 3,047,393; and 4,960,689, have also been employed.
Aromatic, heterocyclic, and acyclic disulfides which do not have labile sulfur or sulfide, such as those described in U.S. Pat. Nos. 1,962,133; 2,465,149; 2,756,145; 3,043,696; 3,057,725; 3,062,654; 3,128,186; and 3,563,754, have been used primarily as emulsion melt additives, i.e. being introduced into already (spectral) chemically sensitized silver halide emulsions prior to coating. U.S. Pat. No. 3,397,986 discloses Bis(p-acylamidophenyl)disulfides as useful antifoggants added before or after any optically sensitizing dyes. However, the use of optically sensitizing dyes during chemical sensitization was not readily known in the art until their widespread use during tabular shaped silver halide emulsion sensitization. U.S. Pat. No. 3,397,986 and the others cited previously did not anticipate the utility of these non-labile disulfides during the sensitization of silver halide emulsions, either with or without optically sensitizing dyes. The prior art use of these disulfides as melt additives does decrease fog and stabilize against fog during aging of coated emulsions, but when used in this manner also decreases sensitivity and requires the use of additional stabilizers like azaindenes, such as described in U.S. Pat. No. 3,859,100.
There is a continuing need for improved methods of preventing fog in photographic elements without severely impacting sensitivity. In accordance with this invention, it has been found that the addition of a particular class of non-sensitizing dichalcogenides to a silver halide emulsion immediately before or during (spectral) chemical sensitization gives lower fog without a concomitant large loss in sensitivity. It has also been found that equivalent fog reduction can be obtained with much less dichalcogenide when the dichalcogenide is used during sensitization, rather than as a melt additive, and that less or no latent image destabilization occurs. Additionally, less loss in sensitivity occurs after aging of the coated emulsions.